This application claims the priorities of Japanese Patent Application No.2000-223684 filed on Jul. 25, 2000, and No.2000-222453 filed on Jul. 24, 2000, which are incorporated herein by reference.
The present invention relates generally to a belt-type continuously variable transmission comprising a V belt wound around drive and driven pulleys whose widths are variable, and particularly to a shift control system which controls thrusts directed axially onto the drive and driven pulleys for the speed ratio change of the belt-type continuously variable transmission.
Many types of such transmissions and shift control systems have been proposed, and some are now in practical use. A typical belt-type continuously variable transmission comprises a drive-side actuator, which is used for adjusting the width of the drive pulley (in axial thrust control), and a driven-side actuator, which is used for adjusting the width of the driven pulley (in axial thrust control). In the speed ratio change of the transmission, these actuators are controlled to act on the respective pulleys axially with thrusts that are appropriate for the drive and driven pulleys to achieve their proper pulley widths.
For the purpose of achieving automatic speed ratio change, various belt-type automatic transmissions have been proposed with shift control systems that are designed to control the thrusts of such actuators in correspondence to the driving condition of the vehicle. For example, such a belt-type automatic transmission is disclosed in Japanese Laid-Open Patent Publication No. H9(1997)-72397. In this transmission, for the speed ratio change, the axial thrust of one pulley is controlled to a target thrust value while the thrust of the other pulley is adjusted to a value that is a product of the target thrust value and a pulley thrust ratio or to a value that is the sum of a value which corresponds to the ratio and a value which corresponds to the deviation of the rotational speed of the engine.
Various apparatus or devices and methods have been also proposed to control the speed ratio change of respective belt-type continuously variable transmissions. However, these prior-art systems and methods have experienced problems of inferior responsivity and convergency because of constant gains seen in control feedbacks for both upshifts and downshifts or of inappropriate parameters set as feedback.
It is an object of the present invention to provide a shift control system which can control the axial thrusts of the pulleys to optimal values for the speed ratio change of a belt-type continuously variable transmission.
It is another object of the present invention to provide a shift control system which can control the axial thrust of the pulleys to least values necessary for the speed ratio change of a belt-type continuously variable transmission.
To achieve these objectives, the present invention provides a shift control system which controls the axial thrusts generated by drive-side and driven-side actuators (for example, the drive-side cylinder chamber 6 and the driven-side cylinder chamber 9 described in the following preferred embodiment), respectively, to vary the widths of the drive and driven pulleys for achieving a shift to a target speed change ratio. The shift control system comprises contact-circle enlarging pulley determination means (for example, refer to Step S51 described in the following embodiment) and additional thrust calculation means (for example, refer to Step S5 described in the following embodiment). The contact-circle enlarging pulley determination means determines which pulley, i.e., the drive pulley or the driven pulley, is to be the contact-circle enlarging pulley whose diameter of the belt-contact circle is enlarged during the shift, and the additional thrust calculation means calculates a shifting additional thrust which is added for the contact-circle enlarging pulley to shift the transmission to the target speed change ratio. Preferably, the shifting additional thrust is set inversely proportional to the running speed of the V belt.
While the speed of the V belt is high, if the pulley widths were varied quickly, then the resulting shift would be too rapid. However, if the speed of the shift is controlled in inverse proportion to the running speeds of the V belt, then a shift control appropriate for comfortable travelling is possible. Therefore, the shift control system according to the present invention sets the shifting additional thrust inversely proportional to the running speed of the V belt and realizes a good shift control.
Furthermore, it is preferable that the shifting additional thrust be set proportional to the difference between the diameter of the contact circle of the contact-circle enlarging pulley at present and the diameter of the contact circle of the contact-circle enlarging pulley after the shift to the target speed change ratio. The larger the diameter difference between the belt-contact circles of the contact-circle enlarging pulley before and after the shift, faster the speed of the shift being required. If the speed of the shift is controlled proportionally to the diameter difference between the belt-contact circles, then a shift control appropriate for comfortable travelling is possible. Therefore, the shift control system according to the present invention sets the shifting additional thrust proportional to the diameter difference between the belt-contact circles and realizes a better shift control.
The shift control system can also comprise belt-speed calculation means, which calculates the running speed of the V belt, and first gain coefficient calculation means, which calculates a first gain coefficient in inverse proportion to the running speed of the V belt. In this case, the additional thrust calculation means uses the first gain coefficient for calculating the shifting additional thrust.
Furthermore, the shift control system may comprise belt-speed calculation means, which calculates the running speed of the V belt, and first gain coefficient calculation means, which calculates a first gain coefficient in inverse proportion to the running speed of the V belt, from the rotational speed of the drive pulley and the current speed change ratio. Then, the additional thrust calculation means uses the first gain coefficient for calculating the shifting additional thrust.
According to the present invention, the shifting additional thrust, which is added for the contact-circle enlarging pulley to achieve a shift to a target speed change ratio, can be set proportional to the difference between the diameter of the contact circle of the contact-circle enlarging pulley at present and the diameter of the contact circle of the contact-circle enlarging pulley after the shift to the target speed change ratio.
While the diameter difference between the belt-contact circles of the contact-circle enlarging pulley before and after a shift is relatively large, then the shift needs to be carried out comparatively fast. However, if the speed of the shift is controlled proportionally to the diameter difference, then the shift control can be executed smoothly. Therefore, the shift control system according to the present invention sets the shifting additional thrust proportional to the diameter difference and realizes a good shift operation.
Also, the shift control system may comprise diameter-change calculation means, which calculates the difference between the diameters of the contact circles of the contact-circle enlarging pulley at present and after a shift, and second gain coefficient calculation means, which calculates a second gain coefficient in proportion to the difference between the diameters of the contact circles. In this case, the additional thrust calculation means uses the second gain coefficient for calculating the shifting additional thrust.
It is preferable that the difference between the diameters of the contact circles of the contact-circle enlarging pulley at present and after a shift to a target speed change ratio be calculated from the current speed change ratio and the difference between the rotational speeds of the drive pulley at the current speed change ratio and at the target speed change ratio.
The above described features of the shift control system are realized specifically in the following arrangement. A shift control system according to the present invention controls the axial thrusts generated by drive-side and driven-side actuators (for example, the drive-side and driven-side cylinder chambers 6 and 9 described in the following embodiment), respectively, to vary the widths of the drive and driven pulleys for achieving a shift to a target speed change ratio. The shift control system comprises belt-speed calculation means (for example, refer to Step S52 described in the following embodiment), which calculates the running speed of the V belt; contact-circle enlarging pulley determination means (for example, refer to Step S51 described in the following embodiment), which determines whether the drive pulley or the driven pulley is to be the contact-circle enlarging pulley; diameter-change calculation means (for example, refer to Step S54 described in the following embodiment), which calculates the difference between the diameters of the contact circles of the contact-circle enlarging pulley at present and after the shift; slip prevention thrust calculation means (for example, refer to Step S1 described in the following embodiment), which calculates a slip prevention thrust necessary for transmitting the power through the V belt without slip between the V belt and the drive and driven pulleys; speed ratio maintaining thrust calculation means (for example, refer to Step S2 described in the following embodiment), which calculates a speed ratio maintaining thrust that is added to the slip prevention thrust, for maintaining the current speed change ratio in constant condition and for transmitting the power without any slip of the belt; and additional thrust calculation means (for example, refer to Step S5 described in the following embodiment), which calculates a shifting additional thrust that is added for the contact-circle enlarging pulley to shift the transmission to the target speed change ratio, the shifting additional thrust being set inversely proportional to the running speed of the V belt, which is determined by the belt-speed calculation means, and set proportional to the difference between the diameters of the contact circles of the contact-circle enlarging pulley.
While the speed of the V belt is high, if the pulley widths were varied quickly, then the resulting shift would be too rapid. However, if the speed of the shift is controlled in inverse proportion to the running speed of the V belt, then a shift control appropriate for comfortable travelling is possible. Also, the larger the diameter difference between the belt-contact circles of the contact-circle enlarging pulley before and after the shift, faster the speed of the shift being required. However, if the speed of the shift is controlled proportionally to the diameter difference between the belt-contact circles, then a shift control appropriate for comfortable travelling is possible. Therefore, the shift control system according to the present invention sets the shifting additional thrust inversely proportional to the running speed of the V belt and proportional to the diameter difference between the belt-contact circles and thereby realizes an optimal shift control.
Preferably, a first gain coefficient that is inversely proportional to the running speed of the V belt, which is determined by the belt-speed calculation means, is calculated from the rotational speed of the drive pulley and the current speed change ratio, and a second gain coefficient that is proportional to the difference between the diameters of the belt-contact circles is calculated from the current speed change ratio and from the difference between the rotational speeds of the drive pulley at the current speed change ratio and after the shift to the target speed change ratio. Then, the shifting additional thrust can be calculated on the basis of the first and second gain coefficients. As the calculation of the first and second gain coefficients is relatively simple and easy, the shifting additional thrust can be calculated easily.
Also, for varying the widths of the drive and driven pulleys and achieving a shift to a target speed change ratio, a shift control system may comprise contact-circle enlarging pulley determination means (for example, refer to Step S51 described in the following embodiment), which determines whether the drive pulley or the driven pulley is to be the contact-circle enlarging pulley whose diameter of the belt-contact circle is enlarged during the shift; slip prevention thrust calculation means (for example, refer to Step S1 described in the following embodiment), which calculates a slip prevention thrust necessary for transmitting the power through the V belt without slip between the V belt and the drive and driven pulleys; and additional thrust calculation means (for example, refer to Step S5 described in the following embodiment), which calculates a shifting additional thrust that is added for the contact-circle enlarging pulley to shift the transmission to the target speed change ratio. In this case, for the shift, the axial thrust for the contact-circle contracting pulley is set to the slip prevention thrust, and the axial thrust for the contact-circle enlarging pulley is set to a thrust that is calculated by adding to the shifting additional thrust a product which is obtained by multiplying the slip prevention thrust by the ratio of the thrusts for the drive and driven pulleys, respectively, obtained in constant condition.
Also, a shift control system may comprise contact-circle enlarging pulley determination means (for example, refer to Step S51 described in the following embodiment), which determines whether the drive pulley or the driven pulley is to be the contact-circle enlarging pulley whose diameter of the belt-contact circle is enlarged during a shift to a target speed change ratio; slip prevention thrust calculation means (for example, refer to Step S1 described in the following embodiment), which calculates a slip prevention thrust necessary for transmitting the power through the V belt without slip between the V belt and the drive and driven pulleys; and additional thrust calculation means (for example, refer to Step S5 described in the following embodiment), which calculates a shifting additional thrust that is added for the contact-circle enlarging pulley to achieve the shift. In this shift control system, for the shift, the axial thrust for the contact-circle enlarging pulley may be set to the slip prevention thrust, and the axial thrust for the contact-circle contracting pulley may be set to a thrust that is calculated by multiplying the result obtained by subtracting the shifting additional thrust from the slip prevention thrust by the ratio of the thrusts for the drive and driven pulleys, respectively, obtained in constant condition.
In this way, the thrusts applied to the drive and driven pulleys in the shift control are always minimized with the smaller of these thrusts being the slip prevention thrust, and the magnitude of the larger thrust is determined with respect to the smaller thrust for achieving a desired shift. As a result, a relatively small energy is required to generate the thrusts applied to the drive and driven pulleys in the shift control, and fuel efficiency is improved.
For a more specific shift control system according to the present invention, the following arrangement may be adopted. A shift control system according to the present invention controls the axial thrusts generated by drive-side and driven-side actuators (for example, the drive-side and driven-side cylinder chambers 6 and 9 described in the following embodiment), respectively, to vary the widths of the drive and driven pulleys for achieving a shift to a target speed change ratio. This shift control system comprises contact-circle enlarging pulley determination means (for example, refer to Step S51 described in the following embodiment), which determines whether the drive pulley or the driven pulley is to be the contact-circle enlarging pulley whose diameter of the belt-contact circle is enlarged by the adjustment of the width thereof during the shift; slip prevention thrust calculation means (for example, refer to Step S1 described in the following embodiment), which calculates a slip prevention thrust necessary for transmitting the power through the V belt without slip between the V belt and the drive and driven pulleys; speed ratio maintaining thrust calculation means (for example, refer to Step S2 described in the following embodiment), which calculates a speed ratio maintaining thrust that is added to the slip prevention thrust of either the drive pulley or the driven pulley, for maintaining the current speed change ratio in constant condition and for transmitting the power without any slip of the belt; additional thrust calculation means (for example, refer to Step S5 described in the following embodiment), which calculates a shifting additional thrust that is added for the contact-circle enlarging pulley to shift the transmission to the target speed change ratio; basic shift control thrust calculation means (for example, refer to Step S61 described in the following embodiment), which calculates basic shift control thrusts from the slip prevention thrust, the speed ratio maintaining thrust and the shifting additional thrust, the basic shift control thrusts being to be applied to the drive and driven pulleys for the shift to the target speed change ratio; and shift control thrust correction means (for example, refer to Step S63 and Step S65 described in the following embodiment), which executes a correction in which the basic shift control thrusts are reduced until the thrust for either the drive pulley or the driven pulley becomes equal to the slip prevention thrust with the thrust for the other pulley being equal to or greater than the slip prevention thrust.
In this shift control system, the basic shift control thrusts are calculated from the slip prevention thrusts, the speed ratio maintaining thrust and the shifting additional thrust. If the basic shift control thrusts are applied for the shift control, then a shift to a target speed change ratio can be executed appropriately without any slip between the V belt and the pulleys. However, the basic shift control thrusts calculated in this way can become greater than the slip prevention thrusts for both the drive and driven pulleys. If such a case occurs, then, both the thrusts are reduced until either reaches the respective slip prevention thrust. The thrusts reduced in this way can be used for the shift control to achieve the same result. As a result, the levels of the thrusts can be reduced as a whole. According to the present invention, this reduction correction is executed by the shift control thrust correction means, so that a relatively small energy is required to generate the thrusts applied to the drive and driven pulleys for the shift control. This improves fuel efficiency.
As the basic shift control thrusts are calculated from the slip prevention thrusts, the speed ratio maintaining thrust and the shifting additional thrust, preferably, the shift control system may be arranged to execute a correction in which the thrust that corresponds to the speed ratio maintaining thrust is subtracted from the basic shift control thrusts or in which the thrust that corresponds to the shifting additional thrust is subtracted from the basic shift control thrusts.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.